1.
Ultrasound Physics & Knobology
Hong Kong College of Emergency Medicine
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2.
What is Ultrasound?
Human sensitivity: 20 - 20,000Hz
Ultrasound: > 20,000Hz
Diagnostic Ultrasound: 2.5 - 14 MHz
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3.
Definition/ Terminology
Cycle
Frequency: cycles per second
Wave length
Period
Amplitude
Compression - area of high density
Rarefication - area of low density
velocity = λ x ƒ
= constant for a given medium
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4.
Variation in particle density at one point as a function of time
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5.
A, Initial observation.
B, After a short time, regions of high & low density are displaced to the right
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6.
Ultrasound Production
Piezoelectric effect:
– Crystals vibrate at given frequency when an alternating
current is applied
– Crystal acts as speaker and microphone
Continuous mode:
– therapeutic systems
– continuous-wave Doppler (CW)
Pulsed-echo mode:
– all other applications
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7.
Continuous-wave output
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8.
Pulsed Echo
Signal generation = only ~1% of the entire
pulse cycle
On times; off times
Time of signal return proportional to
distance traveled
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9.
Pulsed wave output
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10.
Pulsed Echo
PRF: pulse repetition frequency
– Very important in Color Doppler
SPL: spatial pulse length
= wavelength x no. of cycles
Maximal resolution = 0.5 SPL
– The smallest distance between 2 points that
USG can delineate
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11.
SPL with maximal resolution. Two objects (vertical lines) are
separated by 0.5 SPL. The echo from each interface is shown
by dashed lines. The objects are just resolvable.
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12.
Pulse length shortened by increasing the frequency.
A The four-cycle pulse from a low-frequency transducer includes both
objects within the SPL.
B The four-cycle pulse from a high-frequency transducer has a shorter
spatial pulse length and can resolve objects located more closely
together.
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13.
Ultrasound Transmission
1540 m/s
– Velocity assumed the same for all tissue in calculation
(which is not totally true)
Acoustic impedance
Attenuation
Reflection
Refraction
Scatter: objects irregular or smaller than the
ultrasound beam
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14.
Windows
Getting a good acoustic window is
important for scanning accuracy and image
quality
Solid organ or fluid filled area
– Liver
– Urinary bladder
– Spleen
– Fluid filled stomach (for pancreatic scan)
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15.
Resolution
Ability to delineate between two different
objects
Axial resolution:
– high frequency
= shorter SPL
= better axial resolution but lower penetration
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16.
Resolution
Lateral Resolution
– Sound beam: width of the crystal
– Compressed down the pathway
– Near field
– Focal zone: best lateral resolution
– Far field
– Dead zone: distance between the transducer
face and the first identifiable echo
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17.
Resolution
Temporal Resolution
– Frame per second
– Multiple focal zones
• decreases frame rate
• decrease temporal resolution
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18.
Diagram illustrating Axial Resolution
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19.
Diagram illustrating Lateral Resolution
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20.
Modes
A mode: amplitude
B mode: brightness
Real time (frames/sec)
M mode: motion
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21.
Doppler Ultrasound
Continuous wave (CW)
– Seldom used, not a/v in most AED machines
Pulsed wave (PW)
Color Doppler
Duplex Doppler
– Putting Color Doppler on top of Grey-scale B
mode
Power Doppler
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22.
Transducers
Formats
– linear - rectangular field of view
– sector - pie-shaped field of view
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23.
In sector scanning, the
resolution becomes poorer
at increasing depth.
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24.
Transducers
Mechanical Probe: seldom used now
Electronic Probe:
– Linear array transducers
• piezoelectric elements linearly arranged
• sequentially activated to produce an image
– Phased array transducers
• smaller scanning surface (foot print)
• good for echocardiography
• more expensive
• elements are activated with phase differences to allow steering
of the ultrasound signal
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25.
Linear array image of testicle
Linear array transducer
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26.
Convex array image of RUQ
Convex array transducer
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27.
Phased array (cardiac) image
Phased array transducer
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28.
Scanning Skills
Try different windows
A lot of gel
Transducer movements:
– Rotate
– Angle (Tip-toe)
– Pivot (fan-shaped movement)
Transducer placement basics:
– longitudinal view
– transverse view
– coronal view
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29.
Knobology
Power
Gain
TGC; DGC; STC
Dynamic range
Depth
Zoom/magnification
Freeze
Calculations
Print
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30.
Image Optimization
Adjust the following in order:
Depth
Focus
TGC
Zoom
Gain
Dynamic Range (contrast)
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31.
Depth
Start with higher “Depth”
Decrease “Depth” to put area of interest
at ¾ depth of screen
Leave a small area behind to observe
useful artifacts e.g. shadowing,
enhancement
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32.
Depth (too much !)
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33.
Depth (too little !)
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34.
Depth (just right !)
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35.
Focus
Focal zone = best lateral resolution
Adjust focus to the level of the point of
interest
Most modern machines can have multiple
focal zones
Increase number of focal zone → decrease
frame rate
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36.
Focus (out focus !)
Focus
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37.
Focus (good !)
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38.
Focus (multiple !)
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39.
TGC
Adjust TGC to obtain a smooth grey-scale
picture
Transit of TGC knobs should be gradual
TGC = DGC = STC
– TGC: time gain compensation
– DGC: distance gain compensation
– STC: spatial time compensation
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40.
Zoom
Zoom in to magnify point of interest
Write-zoom: improve image quality
Read-zoom: only magnification
Useful situation:
– Measurement of CBD
– M-mode documentation of foetal pulsation
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41.
Zoom
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42.
Gain
Optimize gain to obtain most details
Most beginners use too high gain
High gain can obscure many details
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43.
Gain (too high !)
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44.
Gain (too low !)
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45.
Gain (that’s right !)
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46.
Dynamic Range
How white is white and how black is
black?
Adjust Dynamic Range for the contrast
High “Dynamic Range”
= Low “Contrast”
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47.
Dynamic Range (too low !)
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48.
Dynamic Range (too high !)
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49.
Image Descriptions
Anechoic
Echogenic
Hyperechoic Vs Isoechoic Vs Hypoechoic
Interface
Noise
Mirror image
Shadow
Cystic Vs Sonolucent
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50.
Artifacts
Shadowing
Enhancement
Edge artifact
Side-lobe artifact
Reverberation
Comet tail
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51.
Shadowing and Enhancement
Longitudinal image of the gallbladder. Shadowing artifacts are caused by
gallstones. Enhancement is seen as a result of the fluid-filled gallbladder.
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52.
Edge Artifact
Refraction artifacts at the
edges of ruptured patellar
tendon.
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53.
Side-lobe Artifact
Side lobes (shaded regions) at various
angles with respect to the main beam.
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54.
Reverberation Artifact
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55.
Reverberation Artifact
Reverberation artifact in the bladder
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56.
Comet Tail Artifact
FB
Comet tail artifact (arrow) at the back of a metallic FB.
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57.
Guide to Image Acquisition
Relearn anatomy
Correct machine and transducer
Master the gain, power, and TGC buttons
Maximize the gain before increasing the power
TGC should be smooth
Boundaries of organs should be visualized
Go from wider view and zoom in
Acquire images in at least two planes
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58.
Infection Control
Wipe off and disinfect the transducer after
use
Wear glove on the scan hand
Cover TVS probe with condom
Dispose used condom appropiately
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59.
Maintenance
DO
– switch off machine after use
– ‘freeze’ when not scanning
– handle transducers with care
– clean transducers after use with soft towel or
tissue paper
– disinfect transducers after use
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60.
Maintenance
Don’t
– Drop, shock or knock transducers
– Disconnect or connect transducers when power
is ‘on’ (Hot plug is OK on some new machine)
– Leave extension cord on the floor to be rolled
over by the machine
– Leave extension cord in tension or in acute
angles. The connecting points will be disrupted.
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61.
Biological Effects
Cavitation
– rarefication phase
– liquid to gas
– stable Vs collapse
Heat production
There is so far no firm evidence of Bio-
hazard of Diagnostic Ultrasound to human
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62.
Biological Effects
AIUM statement on clinical safety 1983
No confirmed biological effects on patients or
instrument operators caused by exposure at intensities
typical of present diagnostic ultrasound instruments
have ever been reported. Although the possibility
exists that such biological effects may be identified in
the future, current data indicate that the benefits to
patients of the prudent use of diagnostic ultrasound
outweigh the risks, if any, that may be present.
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63.
Biological Effects
ALARA principle:
As Low As Reasonably Achievable
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64.
The End
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